1. Field of the Invention
The present invention relates to a method for bonding ceramics to each other or a ceramic to a metal. Particularly, it relates to a method for bonding which is suitable for bonding of structural parts of a machine, electronic parts or the like.
British Patent No. 761045 discloses a method for bonding alumina to a metal. This method comprises oxidizing copper, placing the oxidized copper (cuprous oxide: Cu.sub.2 O) on a ceramic substrate, heating both at a temperature higher than the melting point of copper (1083.degree. C.) and lower than the melting point of copper oxide (1230.degree. C.) (to form copper oxide on the copper) and reacting a eutectic crystal consisting of copper oxide and liquid copper with the substrate to thereby bond the alumina to the metal.
The above method requires heating the components at a temperature higher than 1083.degree. C., and lower than 1230.degree. C., so that the difference in thermal expansion between alumina and a metal causes thermal stress when cooling the bonded components. Therefore, the alumina tends to crack, thus bringing about lowering of reliability with respect to strength. What is worse, the metal is distorted when heated. U.S. Pat. No. 4,037,027 discloses a method which comprises heating components at a low temperature at which an inserting material is not melted and pressing them, i.e., a solid phase bonding method. According to this method, no large thermal stress is generated, because the heating temperature is low. However, because the inserting material is not melted and an oxide layer is formed on the surface of the inserting material, the reaction between the inserting material and components to be bonded is slow, so that a high bond strength is difficultly obtained for a short time. Further, a high pressure is required to contact the bonding surfaces with each other tightly.